DRILLING AT PACMANUS: ANATOMY OF A DACITE-HOSTED, ACTIVE HYDROTHERMAL SYSTEM IN A RIFTED BACKARC BASIN

The ~1700 mbsl PACMANUS hydrothermal system extends 3 km along the crest of Pual Ridge, a submarine dacitic volcano in the eastern Manus backarc basin, PNG. It includes active sulfide chimney fields notably rich in Cu-Zn-Ag-Au, a field of low-T diffuse venting with exposures of altered rhyodacite, and deposits of Fe-Mn-Si oxide.

ODP Leg 193 drilled to 387 mbsf under the Snowcap low-T vent site, and 206 mbsf at the Roman Ruins high-T chimney site, revealing contrasted alteration and mineralization patterns. Both sites display pervasive clay-dominated alteration beneath a thin cap of fresh lava. Preservation of igneous fabrics and Zr/TiO2 ratios define a “layer-cake” sequence of flows ranging from andesite to rhyodacite, but no intrusives. Paleoseafloor horizons but no fossil exhalative deposits were recognized.

The subsurface alteration and mineralization reflect a single though multiphase hydrothermal event imposed after major construction of the volcano. Assemblages with cristobalite pass into quartz-rich rocks at depth under both sites, the transition lying higher at Roman Ruins. Illite and chlorite, the principal clay minerals, show no obvious passage into deeper, higher temperature assemblages. They are accompanied by widespread disseminated pyrite and, under Roman Ruins, by hydrothermal K-feldspar. Localized bleaching to pyrophyllite occurs in the cristobalite zone under Snowcap. Breccias, and anhydrite veins controlled by late fractures are common, especially at Snowcap. Rare massive pyritic sulfides (some with significant chalcopyrite and gold) were cored under Roman Ruins, mostly in a stockwork zone extending ~120 mbsf to a possible fault underlain by unmineralized and less altered lavas.

Subsurface mixing between seawater and a partly magmatic hydrothermal fluid under PACMANUS was accompanied by boiling, possibly leading to segregation of Cu-Au and Pb-Zn-Ag. Most Zn and some Pb in seafloor chimneys were likely leached from the volcanic edifice during alteration, whereas Cu and Au derive mainly from the postulated subjacent high-T reaction zone and/or from a magma body ~2 km below the crest of Pual Ridge, as indicated by borehole temperatures and seismic imagery (KORDI). Tectonic rupture of the fresh dacite cap to the “PACMANUS pressure cooker” governs location of the seafloor chimney fields.